This invention relates to an improved continuous process for the carbonylation of ethylene.
The carbonylation of ethylenically unsaturated compounds using carbon monoxide in the presence of an alcohol or water and a catalyst system comprising a group 6, 8, 9 or 10 metal, for example, palladium, and a phosphine ligand, for example an alkyl phosphine, cycloalkyl phosphine, aryl phosphine, pyridyl phosphine or bidentate phosphine, has been described in numerous European patents and patent applications, for example EP-A-0055875, EP-A-04489472, EP-A-0106379, EP-A-0235864, EP-A-0274795, EP-A-0499329, EP-A-0386833, EP-A-0441447, EP-A-0489472, EP-A-0282142, EP-A-0227160, EP-A-0495547 and EP-A-0495548. In particular, EP-A-0227160, EP-A-0495547 and EP-A-0495548 disclose that bidentate phosphine ligands provide catalyst systems which enable high reaction rates to be achieved. C3 alkyl bridges between the phosphorus atoms are exemplified in EPO495548 together with tertiary butyl substituents on the phosphorus. WO96/19434, WO 01/68583, WO 98/42717, WO 03/070370, WO 04/103948 and WO 05/082830 relate to developments in these bidentate ligands and their applications.
In particular, WO96/19434 relates to an improved carbonylation of ethylene in the presence of methanol co-reactant to produce methyl propionate in the presence of a bidentate phosphine ligand and a palladium catalyst. The reaction is advantageously carried out in the presence of an acid such as a sulphonic acid. It is advantageous to include acid in the catalyst feed because this allows a more stable metal(0) compound to be used so that the catalytically active metal cation can be generated in situ by the presence of acid prior to the reactor. Unfortunately, the design of a continuous process is hindered by the presence of this acid because whereas the reactants are consumed by the reaction and the palladium and the ligand catalyst eventually decompose in the reactor, the acid will tend to accumulate in the reactor and potentially lead to a steady and unwanted pH rise. The catalyst system can be removed in the product stream, be separated subsequently and neutralised before recycling but this leads to potential contamination of the catalyst system with base salts. Neutralisation of the product stream with base followed by removal of the metal and ligand is an even more complex series of separation steps. Accordingly, it is necessary to address the problem of acid build up in the reactor without contaminating the catalyst system or the product.